1. Field of Invention
This invention pertains generally to processes for recovering silver from waste materials and, more particularly, to processes for recovering silver from paper.
2. Description of the Related Art
U.S. Pat. No. 3,928,253, issued to Thornton et al., discloses a procedure for separating and recovering clean polyester separately from the materials with which it is coated and for recovering desired values present in the coating materials, whereby the recovered polyester and other valuable recovered materials may be recycled thereby avoiding, or minimizing pollution from such materials. When the starting material is a polyester photographic film, the treatment includes detaching the contaminants from the polyester film base by contacting the polyester photographic film scrap with an aqueous solution of monoethanolamine containing a 2% to 15% by weight of water and having a temperature between 100° C. and 170° C., for a time sufficient to remove both the coating and the subcoating from said base. This treatment generally includes the mechanical separation of coatings in solution using screens and sleeves.
U.S. Pat. No. 4,078,916, issued to Gerber et al., discloses a process in which scrap photographic film comprising at least a four component system involving a polyalkyleneterephthalate film base, an adhesive or subbing layer utilizing a terpolymer consisting chiefly of polyvinylidenechloride and a gelatin layer containing metallic silver or a compound thereof and a variety of contaminants such as causal dirt, dyes and the like is broken down for recovery of the valuable constituents therein including the silver, the polyvinylidenechloride terpolymer, and the polyesters by treating the chopped photographic film at an elevated temperature with a solvent for both the polyester and polyvinylidenechloride values in which solvent both the gelatin and silver values are insoluble, under conditions chosen to insure the quantitative recovery of the silver values. This is solvent-based approach to recover major components as raw materials, with silver as a metallic element. Such solvent-based silver-recovery approaches have a limited effectiveness.
U.S. Pat. No. 4,765,835, issued to Schoenhard, discloses a process in which silver is removed from polymeric film chips such as photographic and X-ray film in a caustic treatment bath which dissolves or separates the saran and gelatin layer from the polymer, particularly polyester, layer. The silver is captured for recovery in a diatomaceous filter medium through which the treatment bath liquid is passed. Preferably liquid passed to the filter is silver-rich liquid skimmed from the top of the treatment bath. Chips and other debris are seived or screened from the liquid before filtering. This approach uses base metal salts such as sodium chloride and potassium chloride, as well as oxidizing agents like sodium hypochlorite, ozone, and hydrogen peroxide. The disclosed process also uses monoethamolamine and morpholine, at least in some embodiments. Additionally, this approach relies upon sieves and screens to separate substrates and coatings, and the process does not reduce the recovered silver.
U.S. Pat. No. 4,799,954, issued to Hochberg, discloses a process for the recovery of silver from exposed or unexposed photographic film using a caustic alkali solution at elevated temperatures with high shear. The photographic film is generally coated with an adhesion promoting layer such as a vinylidine chloride polymer or copolymer, which in turn, is coated with a light-sensitive emulsion layer containing a silver halide. In accordance with the present invention, the photographic film, whether in an exposed and developed or unexposed state, is contacted with a caustic alkali solution. The caustic used can, for example, be sodium hydroxide or potassium hydroxide of which sodium hydroxide is preferred. The concentration of the solution should be at least about 7 wt. percent of the caustic, and maintained at a treatment temperature of about from 140° F. to 200° F. The photographic film, in the hot caustic solution, is subjected to high shear. High shear is used in its conventional sense, that is, mixing sufficient to create turbulent as opposed to laminar flow within the treatment vessel. To promote high shear, the photographic film is preferably in the form of small pieces, so that the turbulence of the film pieces aids in the development of shear within the treatment vessel. Particularly preferred are pieces of film ranging in size from about 2 to about 30 millimeters in the longest dimension. The high shear mixing in the caustic solution is carried out for a period sufficient to reduce the silver halides in the emulsion layer to metallic silver.
U.S. Pat. No. 5,064,466, issued to Hilton, discloses a non-toxic process for the recovery of silver from silver containing photographic emulsion coated paper or film substrates. The process includes the use of a stripping formulation comprised of an alkaline bleach solution which is capable of holding stripped animal protein, silver and silver halide in solution during separation of the substrate materials. The solution containing the silver, silver halide and animal protein is treated with a strong or highly ionized acid which results in controlled precipitation of hydrolyzed animal protein, silver and silver halide. Waste fluids are neutralized with an alkali metal hydroxide, the precipitate is dried to a sludge, carbon content of the sludge is control burned and the residue is smelted with appropriate fluxes in order to achieve silver ingot. This process provides no reduction of silver halides, and the process uses hypochlorite (which always presents a risk of chlorine gas if not used carefully). This process does not recover paper or substrates after stripping.
U.S. Pat. No. 5,286,463, issued to Schwartz, discloses a composition for use in reducing and recovering metals, such as silver, and for use in stripping adhesive polymer layers from various polymer bases, such as polyester, and processes for using same are provided. The composition and variations thereof can be useful in separating an adhesive polymeric layer from the film for recovery of either. The composition comprises a reducing sugar/alkaline solution that is particularly useful for recovering silver from polyester photographic film where a silver halide light-sensitive emulsion layer is adhered to the polyester film by a polymeric adhesive resin, such as resins containing copolymers of polyvinylidene chloride and polyvinyl chloride. This process uses reducing sugars, spraying the composition on paper and then preheating the paper to about 80° C. to effect silver reduction. Paper and coating are then burned to recover silver.
A need is felt for a process for recovering silver from paper that also allows recovery, followed by reuse and repurposing, of the paper substrate.